1. Field of the Invention
This invention relates to surface acoustic wave elements, surface acoustic wave devices, surface acoustic wave duplexers, and to methods of manufacturing surface acoustic wave elements.
2. Related Background Technology
Recently, instead of a filter using dielectrics and a resonator, there are more cases in which a surface acoustic wave filter and a surface acoustic wave resonator, which are types of surface acoustic wave devices, are used for a high frequency circuit of a mobile communication device, such as a portable telephone, a cordless telephone, and the like. This is because the dimension of the surface acoustic wave filter is smaller than that of a dielectric filter, and one in the same size have advantage in the electrical characteristics.
This type of surface acoustic wave device is mainly provided with a piezoelectric substrate, a comb electrode (Inter Digital Transducer, hereafter referred to as “IDT”) obtained by forming an electrode film laminated on the piezoelectric substrate, and a package that accommodates the piezoelectric substrate and the IDT. Lithium niobate (LiNbO3), lithium tantalate (LiTaO3), quartz, or the like is used as a material of the piezoelectric substrate. In particular, when an RF band filter is manufactured, there are many cases in which lithium niobate and lithium tantalate having high electro-mechanical coupling coefficients are used. Lithium niobate has a large electro-mechanical coupling coefficient, so a substrate with a 64°-rotated Y-cut is used. In addition, lithium tantalate has a high electro-mechanical coupling coefficient and a relatively small temperature coefficient of frequency, so a substrate with a 34–44°-rotated Y-cut is used. Furthermore, in many cases, aluminum with a characteristic of low density and low electrical resistivity is used for a material of an electrode film.
As described above, there are many cases in which a surface acoustic wave device is used for a portable telephone or the like in an RF band (800 MHz–2 GHz). Additionally, for example, when a surface acoustic wave device is used in a 1 GHz band, and large repeated stresses, which are proportional to the frequency during operation, are added to the IDT on the piezoelectric substrate, there was a problem that life duration (lifetime) of the surface acoustic wave device is shortened because migration of aluminum atoms due to the repeated stresses is generated, and defects such as hillocks (protrusions), voids (depletions), and the like are generated in the IDT. That is, electrical durability of the IDT (i.e., the electrode film) becomes an extremely important factor for the lifetime of the device. In addition, a deterioration phenomenon of the IDT significantly appears due to the increase in applied voltage in addition to the shift to high frequency operation. Furthermore, in terms of designing the device, as the frequency increases, the electrode film must be made thinner, and the electrode width must be made narrower. As a result, this type of deterioration phenomenon increases.
Meanwhile, with respect to a duplexer that occupies a large space in a high frequency circuit of a portable phone, it is proposed that a conventional dielectric duplexer should be changed to a small surface acoustic wave duplexer. However, this surface acoustic wave duplexer is an extremely small component, and there was a problem of electrical durability against a large electric power applied particularly to the sending side of the duplexer. Furthermore, as an IDT formation area of the surface acoustic wave device is designed to be large, it is possible to increase the electrical durability by reducing an effective electric power density. However, in this case, there is a problem that the device cannot be made smaller than a certain size.
Due to the above-mentioned reasons, improvement of the electrical durability of the IDT of surface acoustic wave devices, i.e., the electrical durability of the electrode film formed on the piezoelectric substrate is strongly demanded.
It is well known that the diffusion of aluminum at crystal grain boundaries is faster than inside the crystal grains. It is considered that controlling the diffusion at the grain boundary has priority. Therefore, it has been considered that the above-mentioned migration of the aluminum atoms due to repeated stress mainly occurs at crystal grain boundaries. This has conventionally been addressed. Because of this, it has been predicted that electrical durability can be significantly improved by reducing or removing crystal grain boundaries of, or, more preferably, monocrystallizing, an aluminum electrode film.
Therefore, Japanese Laid-Open Patent Application 55-49014 discloses a technology in which an electrode film is substantially made to be monocrystalline. An object of such technology is to improve a performance capability of a surface acoustic wave device regardless of a substance constituting a material of the device.
In addition, Japanese Patent Publication 2545983 discloses a technology which applies, as an electrode film of the surface acoustic wave device, an aluminum film of monocrystal or in which the crystal direction is oriented in a constant direction. In this publication, a Y-cut quartz substrate in the range from 25°-rotated Y-cut to 39°-rotated Y-cut is used as a piezoelectric substrate and is deposited at a high speed (film formation speed: 4 nm/seconds) and at a low temperature (substrate temperature: 80° C.), and thereby an orientation film (311) is obtained. This film is an epitaxial film which is similar to monocrystal.
Japanese Laid-Open Patent Application 6-132777 discloses a technology related to an aluminum monocrystal electrode film different from the one disclosed in the above-mentioned Japanese Laid-Open Application 55-49014. This reference discloses that an aluminum monocrystal film can be obtained if a film is formed at an extremely low speed. More specifically, formation of an aluminum monocrystal film on an LST cut quartz substrate according to a vacuum deposition method, formation of an aluminum monocrystal film on a 128°-rotated Y-cut lithium niobate substrate according to the vacuum deposition method, and formation of an aluminum monocrystal film on a 112°-rotated X-cut lithium tantalate substrate according to the vacuum deposition method are possible.
International Publication No. WO 00/74235 discloses that a 38–44°-rotated Y-cut lithium tantalate monocrystal substrate is used as a piezoelectric substrate, an interdigital-type electrode includes a titanium base metal film and an aluminum film formed thereon, and the aluminum film becomes a monocrystal film due to the effect of the titanium base metal film. Furthermore, Japanese Laid-Open Patent Application 2003-101372 discloses a surface acoustic wave device provided with a piezoelectric substrate formed of lithium tantalate or lithium niobate 33°±9°-rotated Y-cut, one of a titanium nitride buffer layer, a multi-layer buffer layer in which titanium nitride and titanium metal are laminated, and an inclined composition buffer layer where the composition changes gradually from titanium nitride and titanium metal, laminated on the substrate, and a monocrystal aluminum laminated on the buffer layer.